Digital networks have been developed to facilitate the transfer of information, including data and programs, among digital computer systems and other digital devices. A variety of types of networks have been developed and implemented using diverse information transfer methodologies. In some networks, such as the well-known Ethernet, a single wire is used to interconnect all of the devices connected to the network. While this simplifies wiring of the network in a facility and connection of the devices to the network, it results in generally slow information transfer, since the wire can only carry information, in the form of messages, from a single device at a time. To alleviate this to some extent, in some Ethernet installations, the network is divided into a number of sub-networks, each having a separate wire, with interfaces interconnecting the wires. In such installations, wires can carry messages for devices connected thereto simultaneously, which increases the number of messages that can be transferred simultaneously. It is only when a device connected to one wire needs to send a message to a device connected to another wire that wires in two or more sub-networks will be used, making them unavailable for use by other devices connected thereto.
To alleviate this, networks have been developed in which communications are handled through a mesh of routing nodes. The computer systems and other devices are connected to various ones of the routing nodes. Since the routing nodes themselves are interconnected in a variety of patterns, a number of paths may be available between pairs of the devices, so that if one path is congested, another may be used. Such an arrangement may result in a network which is more complicated than an Ethernet network, but it can provide substantially higher information transfer rates, particularly if optical fiber is used as the media interconnecting the routing nodes and devices. A problem which may arise with such networks is that, in such networks, a routing node or a device, when it is receiving information from another routing node or device in the network, does not have a mechanism to provide "flow-control" information to the transmitting routing node or device. While this does reduce the cost of a network, it may result in congestion, in which a routing node may receive information at a rate faster than it can transmit it.
This problem has been addressed in one type of network, namely, a network implemented in accordance with the ATM ("Asynchronous Transfer Mode") methodology. In such a network, a "packet" of data is transmitted from a source device to one or more destination devices in a series of "cells." If a routing node detects congestion, such that it is receiving cells faster than it can transmit them, it can make use of several mechanisms. In one such mechanism, identified as "early packet discard," which may be used if a moderate amount of congestion is experienced, the routing node first refuses to accept cells related to any new packets, but it attempts to continue transferring cells associated with packets it has already begin transferring. This may alleviate the congestion downstream of the routing node, or at least provide that it does not increase. However, the congestion may continue increasing to a point where the node activates a second mechanism, identified as "partial packet discard." In the partial packet discard mechanism, if the routing node, due to increased congestion, has to drop one cell for a packet that it has begun transferring, it will continue dropping the cells from the same packet because all of the cells for a packet are required to correctly reassemble the packet at the destination. If the partial packet discard mechanism is activated due to congestion, partial packet discard should reduce it, but the packets which have been discarded must be retransmitted by the source in any case, so the routing nodes's resources used to transfer the cells prior to activation of the partial packet discard mechanism were wasted.